TUESDAY, Jan. 31 (HealthDay News) -- Physicians can boost their chances of finding signs of colorectal cancer in patients with a second flexible sigmoidoscopy test, which is a less invasive procedure than a colonoscopy, a new study shows.

The study looked at the value of following up a negative flexible sigmoidoscopy test -- one in which cancer is not found -- with a repeat test.

However, "you don't find twice as many [signs of trouble], but 20 to 30 percent more. This has to be evaluated against the cost and the small risk of repeating the procedure," said study author Dr. Joel Weissfeld, an associate professor of epidemiology at the University of Pittsburgh.

The study appears in the Jan. 31 issue of the Journal of the National Cancer Institute.

The U.S. National Cancer Institute estimates that colon and rectal cancer kill more than 51,000 people a year in the United States. Screening helps identify cancer before symptoms appear, when it may be easier to treat.

Colonoscopies, which involve threading a scope with a tiny camera through the length of the colon, are the most common screening test for colorectal cancer.

An earlier study reported last June in the journal BMJ found that colonoscopy, the more expensive of the two tests, is significantly better at spotting cancer in older patients and remains the "gold standard."

But some patients choose to undergo flexible sigmoidoscopies, which use a shorter scope and examine half or less of the colon. They used to be the main screening test for colon cancer, but have lost their supremacy to colonoscopies.

Sigmoidoscopies may be more comfortable for some patients and they don't require sedation. But both procedures require patients to undergo the unpleasant cleansing of their bowels with the help of laxatives beforehand.

Sigmoidoscopy "does a pretty good job" of finding signs of colon cancer, said Polly Newcomb, head of the cancer prevention program at the Fred Hutchinson Cancer Research Center in Seattle and co-author of a commentary accompanying the new study.

The research is part of a larger project examining various types of cancer screening. The researchers looked at the experiences of nearly 35,000 people -- aged 55 to 74 -- who underwent two sigmoidoscopies over a period of three to five years.

The alternative procedure increased the number of times that colon cancer or benign tumors were detected by roughly one-third. Physicians detected signs of trouble in about 38 per 1,000 persons after the first screening, and that number grew to almost 50 per 1,000 persons after both screenings.

The detected cancers were in the early stages 80 percent of the time.

While the new study has some limitations, it will help researchers as they try to figure out how often people should get sigmoidoscopies, said Newcomb.

After the age of 50, the American Cancer Society recommends a flexible sigmoidoscopy once every five years or a colonoscopy every 10 years. Other tests are also available, and the organization recommends them at different intervals.

More information

For more about colon cancer, try the U.S. National Library of Medicine.